There is little understanding of how cells achieve and then maintain a given size. This dearth of understanding is due primarily to technical limitations related to imprecise measures of cell size, artifacts from synchronizing cells, and population averaging. We intend to develop a microsystem for cell sizing (MCS) that will - in a single leap - overcome these three technical limitations. The MCS will circulate cells through consecutive modules that i) maintain the pH, nutrient, and gas balance in the medium, ii) allow for the addition of stimuli, iii) measure cell fluorescence from molecular reporters, and iv) measure mass and mass density, and thus volume. Approximately 100 cells will pass through the system and it will be possible to monitor these cells for more than 24 hours. The cells will flow in single file, allowing individual cells to be tracked. Cell divisions that occur within the loop will be identified and the two daughter cells will then be separated by shear flow and tracked individually. The relationship between cell growth and G1 phase is central to understanding the cell cycle and has important ramifications for oncology. For mammalian cells, remarkable gaps exist between the molecular biology of G1 phase, which has been extensively explored, and the basic physiology of G1 phase, which has not. There is currently no understanding of how cell growth and size interface with the two key physiological transitions of G1 phase: the Restriction Point and the exit from G1 phase (G1/S transition). Furthermore, the relative order and dependency of most of the molecular events in G1 phase is not known. By virtue of its capability to continuously measure growth, cell cycle time, and proteins, the MCS could become a popular general platform for single cell experimentation. For instance, the MCS could enable known molecular events (as reported by GFP fusions in live cells and immunostaining in fixed cells) to be correlated to the Restriction Point, G1/S transition, growth curve, cell cycle time, and each other. In the case of quiescent cells, it is not known if size is maintained perfectly or if corrective episodes of growth or autophagy are exhibited. There are many additional uses of the MCS. For instance, precise measurements of cell mass could aid the study of autophagy, the extent of which may be related to cell death decisions. As a sensitive indicator of cell metabolism, individual growth curves could also be used as a read-out for the on-target or off-target effects of drugs. We intend to develop a microsystem for cell sizing (MCS) that will - in a single leap - overcome technical limitations that have stifled research into a classic problem in cell biology: how cells control their size. The MCS will circulate cells through consecutive modules that measure single cell mass, density and fluorescence, and provide nutrients and stimuli. The cells will flow in single file, allowing individual cells to be tracked. Cell divisions that occur within the loop will be identified and the two daughter cells will then be separated by shear flow and tracked individually.